1. Field of the Invention
The present invention relates generally to radiation testing and medical treatment, and, in particular, relates to radiation testing and the medical treatment using gamma radiation sources, and, in greater particularity, relates to radiation testing and medical treatment using 75Selenium.
2. Description of the Prior Art
The use of radioactive sources in present technology is an important feature as compared to x-rays, for example, in that radioactive sources can be tailored to specific uses. These sources may emit particle and/or wave radiation of varying energies in the spectrum and further have half-lives from mere seconds to years. Since some of these elements are extremely reactive, processing these sources into useable products is a critical and complex task to protect users and minimize manufacturing costs.
U.S. Pat. No. 6,875,377 describes the specific problem of combining 75Selenium with an acceptable metal source:                “In the past, 75Selenium sources have been made by encapsulating elemental 74Selenium target material inside a welded metal target capsule. This is irradiated in a high flux reactor to convert some of the 74Selenium to 75Selenium. Typically, target capsules are made of low-activating metals, such as aluminum, titanium, vanadium and their alloys. Other expensive metals and alloys are also possible. The use of these metals ensures that impurity gamma rays arising from the activation of the target capsule are minimized. The 75Selenium is typically located within a cylindrical cavity inside the target capsule in the form of a pressed pellet or cast bead. To achieve good performance in radiography applications it is necessary for the focal spot size to be as small as possible and the activity to be as high as possible. This is achieved by irradiating in a very high neutron flux and by using very highly isotopically enriched 74Selenium target material, typically >95% enrichment.        After the irradiation, the activated target capsule is welded into one or more outer metal capsules to provide a leak-free source, which is free from external radioactive contamination.        Elemental selenium is chemically and physically volatile. It melts at 220° C. and boils at 680° C. It reacts with many metals, which might be suitable as low-activating capsule materials at temperatures above about 400° C.; this includes titanium, vanadium and aluminum and their alloys. Selenium may react explosively with aluminum. This means that careful choice of target capsule material is required and the temperature of the target capsule during irradiation must be kept below about 400° C. to prevent the selenium reacting with, and corroding the target capsule wall. If this occurred, it would increase the focal spot size, distort the focal spot shape and reduce the wall thickness and strength of the target capsule.”        
A solution is offered in U.S. Pat. No. 6,875,377 which is incorporated by reference:                “An embodiment of the present invention is to provide a source having a selenium target composition, which overcomes or ameliorates one or more of the problems associated with the use of elemental selenium, specifically the problems of achieving a thermally stable, non-volatile, non-reactive, high density, stable selenium target which nevertheless contains a very high density of selenium, comparable with the elemental form of the material.        The invention provides; in one of its aspects, a gamma radiation source comprising 75Selenium or a precursor thereof, wherein the selenium is provided in the form of one or more thermally stable compounds, alloys, or mixed metal phases with one or more metals (hereinafter referred to as acceptable metals or an acceptable metal) the neutron irradiation of which does not produce products capable of sustained emission of radiation which would unacceptably interfere with the gamma radiation of 75Selenium.”        
A definition of what is an “acceptable” metal is also provided in U.S. Pat. No. 6,875,377:                “Thus, for example, an acceptable metal, such as vanadium or rhodium, is activated but has no interfering gamma radiation. Molybdenum produces molybdenum-99 that does have interfering gamma radiation, but is very short lived and is therefore also an acceptable metal. Again, Thorium produces palladium-233 [appears to be an error in that thorium produces protactinium-233] having a 27 day half life, but the gamma radiation of palladium-233 is 300-340 keV which is very similar to selenium-75 and therefore acceptable.”        
The above U.S. Patent lists “acceptable metals”:                “Preferably, the said acceptable metal or metals is from the group comprising vanadium, molybdenum, rhodium, niobium, thorium, titanium, nickel, lead, bismuth, platinum, palladium [see not above], aluminum, or mixtures thereof. More preferably, the said acceptable metal or metals comprises one or a mixture of vanadium or molybdenum or rhodium.”        
Accordingly, there is an established need for radiation sources that would complement present sources that would provide additional benefits.